May it be possible that you have the origin layer of the cloning (in the case of my screenshot below "current layer and below) set to "all layers" and therefore the stamp tool picking up something it shouldn't? (like the result of an adjustement layer at the top of the layer stack for example)
Just a heads up, sometimes a unwanted keyboard shortcut can alter these things (it happens to me all the time!). For example when hitting the down and up arrows.
You need to provide more information, what exact version of Photoshop are you using, can you post a screenshot of the Photoshop interface with all the pertinent panels opened
You have at least one layer 🙂 - Always! What is the blending mode of your stamp tool? It should be on "normal" (the first one in the list) unless you specifically want to tweak that... (see the screenshot in my previous answer to have a look at where to find the blending mode (the drop down list named "mode" just before the opacity)
Sorry I am unfamiliar with the language in your UI, so I can't tell you if its' the case or not.